The present invention relates to a laser system and method of forming economical and reliable blind vias in circuit boards and polymer based multichip modules.
Laser drilled blind vias are constructed by positioning and pulsing laser beam radiation over a pre-etched window to remove dielectric material. The use of pre-etched windows as a mask for laser drilling multilayer circuit boards is disclosed in U.S. Pat. No. 4,642,160.
The construction of multilayer circuit boards and the processes used to produce them are well understood. Through vias or holes that interconnect one side of a circuit board completely to the other and that have been made conductive have been the Z-Axis interconnect technology choice for multilayer circuit boards for years. These holes are typically mechanically drilled in stacks on numerically controlled multi-spindle drill machine. The common practice of using leaded components allowed interconnections to be made in the through holes and the need for blind vias was reduced. Surface mount technology where the component leads make interconnections on the surface instead of in the holes actually increases the demand for through vias for electrical interconnections to internal layers in multilayer circuits. As surface mount components increase in pin or lead counts, the pin density become closer. The dense component placement and dense pin count on multilayer circuit boards and polymer based multichip modules creates an interconnect density problem in the Z-Axis. This problem is called via starvation.
One solution for this via starvation is blind via technology as depicted in FIG. 1 and FIG. 2 as via 13 interconnecting down one layer and via 13' interconnecting down to layers two and three. The demand for smaller diameter Z-Axis interconnections coupled with the increased number of interconnections has made the mechanical drill process the most costly sequential process step in the manufacture of multilayer circuit boards.
Integrated circuit technology and component packaging have created a technology demand on the circuit board and polymer multichip module design world. Fine pitch quad flat packs (QFP) (FIG. 15) with 0.305 mm centers and ball grid arrays (BGA) (FIG. 13) lead the packaging challenge as the component footprints are designed with finer I/O pitches as shown by microBGA's in FIG. 14. The preferred solution for interconnecting these dense component footprints on the circuit board demands blind via technology.
Blind via technology has been available for many years but the complex processes needed to produce these blind vias have typically doubled the cost of a circuit board or polymer based multichip module. Four different blind via technologies are known and practiced, but all of these interconnect improvements increase the overall costs of the circuit board and have not openly received broad industry acceptance.
The best known of the four blind via technologies is mechanical drilling which creates the need for sequential layer lamination. Mechanical drilling has been the primary barrier to the growth of blind via technology in the circuit board industry. The economic constraints are due to the lengthy time required to drill small diameter vias and the difficulty to control Z-Axis depths FIG. 11. It is well known that the small holes, less than 0.254 mm diameter, produced by advanced precision computer numerically controlled (CNC) mechanical drills, are drilled at only one panel high. Furthermore, the advanced CNC drill produces approximately 1.5 hits per second per drill spindle. Therefore, a four spindle advanced CNC drill equates to about six hits or vias per second. FIG. 9 and FIG. 10 compares the sequentially laminated mechanically drilled method to the laser drilled method used by the laser system invented in this disclosure. The general processing of the panels for laser drilling in this invention uses the conventional process steps of FIG. 9. Different size vias require different size drill bits and processing time is required to change these drill bits. Another time-consuming characteristic of the current state of the art mechanical drilling is the requirement for the x/y table to come to a complete stop in order to eliminate very small drill bit breakage. This zero movement requirement increases the cost of the CNC drill equipment and is the primary contributor to the average 1.5 hits per second per drill spindle output.
Two batch process blind via technologies are plasma etching and photo via. Plasma etching uses copper as a mask and photovia typically does not. Both plasma etching of blind vias and photovia process are limited to interconnecting one layer down due to the chemical mechanism used for removing dielectric and are called batch processes. Multiple depth processing is accomplished through sequential layer processing, with multiple layer interconnections achieved by Z-Axis daisy chains. This process for making multiple depth blind via interconnections is called sequential buildup technology. Plasma etching demands a dielectric polymer that can readily be chemical etched. This creates some limitations in material selection, similar to the laser processing described in this disclosure, but to a greater extent. Close process control is necessary in order to not overetch the via crater and create a barreled via that is considerably more difficult to metallize.
Even more material limitations are imposed by photovia blind via processing. Photovia materials require a dielectric polymer that has been chemically processed to be photo-sensitive to generally visible light, a ultraviolet light or near ultraviolet light. These photo-sensitive polymers tend to be quite expensive and generally do not meet the requirements for higher performance circuits. Most photovia processing requires additive or semi-additive plating which yields less surface adhesion. This reduced surface adhesion is considered dangerous for most surface mounted components, especially where ball grid array, flip chip and chip scale packaging technology is the chosen component interconnect scheme.
The fourth blind via technology is laser based. Several different laser technologies including Eximer, Nd:YAG (neodymium-doped, yttrium-aluminum-garnet) and CO2 (Carbon Dioxide) have successfully drilled blind vias in circuit boards. The main limitation has been the quantity of panels that can be processed by a single laser system. This is best understood by calculating the blind vias drilled per minute or per second on a given panel. Each panel may have one or many replicated circuit designs that will yield finished circuit boards. Both the Eximer and the Nd:YAG, have extremely small beam sizes--smaller than the diameter of small blind vias. Eximer lasers are quite expensive to run and require ongoing maintenance. Making blind vias of 0.102 mm to 0.203 mm diameter require a complicated trepanning procedure with the Nd:YAG laser. The beam is moved in a spiraling fashion until it removes or ablates the dielectric material from the entire diameter of the via. Another limiting factor with a Nd:YAG beam is that the beam energy is readily absorbed by the copper clad material and, therefore, has to be processed at an extremely low energy level and consequently pulsed multiple times while trepanning. The Nd:YAG laser requires recharging after 60 hours of use.
The sealed RF excited CO2 laser system described in this disclosure has a 20,000 hour life before recharging according to the supplier Synrad, Inc. Another type of CO2 laser system, uses a Transverse Excited Atmospheric (TEA) mechanism to control the pulsing of the CO2 laser. The TEA CO2 laser system produced by Lumonics that is designed for laser drilling blind vias is limited to 150 pulses per second plus with the low 40 watt laser, it is only capable of vaporizing 0.0254 mm of dielectric material with a single pulse. In other words, 0.127 mm of dielectric material would take 5 pulses and the pulse limitation would be at 30 pulses per second.
Early applications of CO2 laser drilling have typically been difficult where the composite materials have vaporization states that are dissimilar. One such example is the common circuit board material of epoxy and woven glass fibers commonly referred to as FR4. In order to vaporize these dissimilar materials, a focused laser is used where the energy needed to effectively remove the material in the via is governed by the success of removing the higher state of material. In order to keep from damaging the mask or blind pin (bottom of the via), the focused laser system must be pulsed several times over a single opening. This multiple pulse condition generally governs and extends the amount of time the beam has to be positioned over the masked window. In most cases, the time to produce multiple pulses demands the motion system be stopped and stabilized to effectively vaporize the laser drilled blind via. To further complicate matters, laser processing of circuit boards grew out of the ceramic hybrid circuit industry where high power focused laser systems with short focal length are the norm.
All of the companies that are producing blind via circuit board laser drilling systems, including Electro Scientific Industries (ESI), Lumonics, Convergent Energy, Mitsubishi, Hitachi and Panasonic, use a Galvanometer or modified Galvanometer for beam positioning and all use a relatively small beam size. The Galvanometer that is used to rapidly position the beam in a small area (from: 0.762 mm.times.0.762 mm up to 50.8 mm.times.50.8 mm), demands that the X/Y positioning table come to a complete stop before using the Galvanometer for local drilling. This adds additional time for stabilizing the Galvanometer, and can add up to 200 seconds to the drilling cycle. In many cases, the drilling time of the laser system embodied in this patent disclosure will be less than the time to move and settle the table with the Galvanometer. The most common alignment for theta is to make an alignment adjustment in software. This theta software adjustment forces the table to continually use two axis before coming to a complete stop. The Galvanometer technology makes incorporating the debris removal technique shown in FIG. 6 and FIG. 20 of this disclosure, difficult if not impossible.
Polymer dielectric materials with improved thermal and electrical characteristics are now available and several are produced with materials that can be categorized as having comparable vaporization states. Examples are epoxy and polyimide resin impregnated non-woven aramid dielectric materials and liquid crystal polymers.
The aramid material produced by DuPont Fibers is called Thermount.RTM. and is the preferred dielectric material for the system and method disclosed in this invention. Multiple material laminators or treaters are processing versions of epoxy and polyamide coated Thermount.RTM.. These U.S. suppliers include NelTec, Arlon, and Polyclad. In addition, Thermount.RTM. provides improved thermal stability for chip on board, flip chip, and chip scale packaging.
Another material that was developed in conjunction with an earlier patent (U.S. Pat. No. 4,642,160) is currently commercially available through AlliedSignal Laminate Systems. This material called RCC.RTM. is a two part epoxy coated copper foil. The epoxy next to the foil is in the C stage or cured and the other epoxy coating is in the B stage or prepreg as noted by the circuit board industry.
A Bismaleimide Triazine (BT) resin coated material that uses a yet to be disclosed non-woven organic fiber similar to the non-woven aramid, is coming out. One such circuit board material is produced by Mitsubishi Gas Chemical Co., LTD (MGC) and is called FoldMax.TM.. This material is also economically laser drilled with the laser system embodied in this invention.
The current limitation in laser drilled diameter is greatly affected by the ability to cost-effectively deposit metal or plating into the blind via's low current density area. The limitation appears to be at the 1:1 window diameter to dielectric depth, especially below 0.076 mm diameter and depths. Several known methodologies can readily plate into the smaller cavities readily produced by laser drilling such as ultrasonic plating, reverse current and laser plating. The most common solution is to use the electroless plating methodology, eliminating low current density conditions with solution movement. This electroless plating method, while capable, does typically add process time and when it eliminates electroplating, it is not practical for large volume panel production. Direct plating, where the polymer surface is made conductive by carbonization, is an acceptable metal conducting method where electroplating can be used to cut down the metallization time.
The laser system and method for rapidly and reliably drilling blind vias at multiple depths described in this invention allows blind vias to be economically introduced into interconnect packaging designs.